Re: [PATCH v2 2/3] mm: khugepaged: fix folio is used after pte_unmap_unlock()

[Vernon Yang]

On Mon, Aug 17, 2026 at 05:33:40PM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 17, 2026 at 05:12:01PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Sat, Aug 15, 2026 at 01:19:23PM +0800, Vernon Yang wrote:
> > > From: Vernon Yang <yanglincheng@xxxxxxxxxx>
> > >
> > > After the page table lock has dropped, the folio can be freed
> > > concurrently. The trace_mm_khugepaged_scan_pmd() is left with
> > > a dangling folio pointer.
> > >
> > > So using the folio_pfn() before dropping the page table lock,
> > > closing use-after-free window.
> > >
> > > Fixes: 7d2eba0557c1 ("mm: add tracepoint for scanning pages")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Vernon Yang <yanglincheng@xxxxxxxxxx>
> > > ---
> > >  include/trace/events/huge_memory.h | 6 +++---
> > >  mm/khugepaged.c                    | 4 +++-
> > >  2 files changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > > index d3572d4ef453..5dc71d292f47 100644
> > > --- a/include/trace/events/huge_memory.h
> > > +++ b/include/trace/events/huge_memory.h
> > > @@ -55,10 +55,10 @@ SCAN_STATUS
> > >
> > >  TRACE_EVENT(mm_khugepaged_scan_pmd,
> > >
> > > -	TP_PROTO(struct mm_struct *mm, struct folio *folio,
> > > +	TP_PROTO(struct mm_struct *mm, unsigned long pfn,
> > >  		 int referenced, int none_or_zero, int status, int unmapped),
> > >
> > > -	TP_ARGS(mm, folio, referenced, none_or_zero, status, unmapped),
> > > +	TP_ARGS(mm, pfn, referenced, none_or_zero, status, unmapped),
> > >
> > >  	TP_STRUCT__entry(
> > >  		__field(struct mm_struct *, mm)
> > > @@ -71,7 +71,7 @@ TRACE_EVENT(mm_khugepaged_scan_pmd,
> > >
> > >  	TP_fast_assign(
> > >  		__entry->mm = mm;
> > > -		__entry->pfn = folio ? folio_pfn(folio) : -1;
> > > +		__entry->pfn = pfn;
> > >  		__entry->referenced = referenced;
> > >  		__entry->none_or_zero = none_or_zero;
> > >  		__entry->status = status;
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index e7830761d3a2..7c8c48577408 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1603,6 +1603,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> > >  	enum scan_result result = SCAN_FAIL;
> > >  	struct page *page = NULL;
> > >  	struct folio *folio = NULL;
> > > +	unsigned long pfn = -1;
> > >  	unsigned long addr;
> > >  	unsigned long enabled_orders;
> > >  	spinlock_t *ptl;
> > > @@ -1778,6 +1779,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> > >  		result = SCAN_SUCCEED;
> > >  	}
> > >  out_unmap:
> > > +	pfn = folio ? folio_pfn(folio) : -1;
>
> You already defaulted the value to -1, better as:
>
> 	if (folio)
> 		pfn = folio_pfn(folio);
>

LGTM, Thanks!

--
Cheers,
Vernon